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- new pybind11::base<> attribute to indicate a subclass relationship
- unified infrastructure for parsing variadic arguments in class_ and cpp_function
- use 'handle' and 'object' more consistently everywhere

Previously, pybind11 required classes using std::shared_ptr<> to derive
from std::enable_shared_from_this<> (or compilation failures would ensue).

Everything now also works for classes that don't do this, assuming that
some basic rules are followed (e.g. never passing "raw" pointers of
instances manged by shared pointers). The safer
std::enable_shared_from_this<> approach continues to be supported.

This modification taps into some newer C++14 features (if present) to
generate function signatures considerably more efficiently at compile
time rather than at run time.

With this change, pybind11 binaries are now *2.1 times* smaller compared
to the Boost.Python baseline in the benchmark. Compilation times get a
nice improvement as well.

Visual Studio 2015 unfortunately doesn't implement 'constexpr' well
enough yet to support this change and uses a runtime fallback.

The cpp_function class accepts a variadic argument, which was formerly
processed twice -- once at registration time, and once in the dispatch
lambda function. This is not only unnecessarily slow but also leads to
code bloat since it adds to the object code generated for every bound
function. This change removes the second pass at dispatch time.

One noteworthy change of this commit is that default arguments are now
constructed (and converted to Python objects) right at declaration time.
Consider the following example:

py::class_<MyClass>("MyClass")
    .def("myFunction", py::arg("arg") = SomeType(123));

In this case, the change means that pybind11 must already be set up to
deal with values of the type 'SomeType', or an exception will be thrown.
Another change is that the "preview" of the default argument in the
function signature is generated using the __repr__ special method. If
it is not available in this type, the signature may not be very helpful,
i.e.:

|  myFunction(...)
|      Signature : (MyClass, arg : SomeType = <SomeType object at 0x101b7b080>) -> None

One workaround (other than defining SomeType.__repr__) is to specify the
human-readable preview of the default argument manually using the more
cumbersome arg_t notation:

py::class_<MyClass>("MyClass")
    .def("myFunction", py::arg_t<SomeType>("arg", SomeType(123), "SomeType(123)"));

Using object class to hold converted object automatically deallocates
object if an exception is thrown or scope is left before constructing
complete Python object.

Additionally added method object::release() that allows to release
ownership of python object without decreasing its reference count.

This gives handle classes a typical pointer semantics with respects to
constness.

On my system (Mac OS with custom Python installation) I had to add the ldflags argument to python-config to get things working.